This project involves the development of retroviral vectors capable of targeting the asialoglycoprotein receptor (ASG-R). It represents the "targeting" portion of a larger effort which has been one of the laboratory's major goals, ie. the development of targetable/injectable vectors, that would be both resistant to inactivation by serum complement and capable of "homing" to specific cell surface receptors, such as the ASG-R. Our strategy for targeting has involved the construction of chimeric envelope glycoproteins, in which the native receptor binding domain of gp70, has been replaced by peptide sequences derived from alpha-1-acid glycoprotein. Stable transduction of HepG2 cells mediated by the ASG-R, has been achieved with vector particles bearing these "chimeric" envelope glycoproteins. We are currently working to improve the efficiency of transduction to the levels that would permit these vectors to be used in vivo. In this regard, vectors expressing human bilirubin-UDP-glucuronyl transferase, from various mouse albumin enhancers have been constructed. The Gunn rat, our animal model for Crigler-Najjar syndrome type I, is the test system for these vectors in vivo. Tissue engineering, when combined with a gene transfer technology, represents another useful approach for delivering therapeutic proteins. Protein C deficiency is another hereditary liver disease, which is invariably fatal in the neonatal period. Vectors which express human Protein C in a variety of cell lines have been constructed. The next stage of this project involves the development of "artificial tissues", that consist of transduced cells which have been established on a three- dimensional matrix in vitro. These tissue substrates will be implanted into rats and evaluated for structure, vascularity, and their ability to secrete biologically-active protein C.